1) Field of the Invention
This invention relates to anion selective membranes useful in electrodialysis apparatus for recovering concentrated acids having low ionic weight anions from dilute, aqueous acid solutions which may also contain, for example, salts, organic compounds, polymers (such as proteins) and/or colloids. Such membranes have a high current efficiency for such anions (generally greater than about 70 percent) and a low electrical resistance in acid solutions (typically less than 30 ohm cm.sup.2).
2) Comparison with the Prior Art
The anion selective membranes of this invention, compared to prior art anion selective membranes, substantially retard the passage of hydrogen ions when in contact with acid solutions and under the influence of a suitable, applied, direct current potential. With the anion selective membranes of this invention, one can efficiently concentrate or deacidify acidic solutions in electrodialysis apparatus.
Commercially available anion selective membranes (such as, for example, those described in U.S. Pat. No. 4,231,855) permit substantial passage of hydrogen ions when they are used for electrodialysis of acid solutions, resulting in low anion current efficiency and therefore high capital and operating costs. For example when typical prior art anion selective membranes separate 0.5 and 1.0 normal solutions of hydrochloric acid at room temperature, then more than 30 percent of the electric current can be carried by hydrogen ions, i.e. less than 70 percent of the current carried by chloride ions. Such disadvantages have severely limited the applications of electrodialysis for the processing of acidic solutions.
The novel membranes of the present invention are useful for the electrodialytic recovery of acid solutions which become "spent", i.e. when, through use, the salt concentration becomes too high and/or the acid concentration is or becomes too low. Such acid/salt solutions are encountered, for example, in the metal finishing, pickling, plating and mining industries. At present such acid/salt solutions often constitute waste disposal problems in such industries since there are few, if any, efficient processes for recovering such acid at useful concentrations from such solutions.